In recent years, the popularity of microwave ovens in the household has increased dramatically. This popularity is due, in part, to the ease and speed of microwave cooking of many foods, compared to conventional cooking. As the number of households having microwave ovens increases, the demand for prepared foods adapted for microwave use also increases. However, microwave cooking of some types of foods has experienced some consumer resistance and dissatisfaction. The characteristics and nature of microwave cooking are substantially different from conventional convection heating, and therefore some foods do not cook well in the microwave oven.
One of the more notable disadvantages of microwave cooking is the inability to produce the highly desirable crispness and/or brown color on the surface of the food. The crisping and brown color are particularly desirable on food substrates such as battered and/or breaded food pieces (e.g., fish and chicken portions), breads, batter based baked goods, and pastries. Microwave cooking even for extended times does not raise the surface temperature of the food to a high enough temperature, for a period of time long enough, to brown or to crispen the food.
The primary reason for such failure is that both crispening and browning generally require a temperature of 300.degree. F..gtoreq.. Such temperatures, of course, are well above the boiling temperature of water and even the elevated boiling temperatures of concentrated aqueous solutions. As food products are microwave heated without assistance of a microwave susceptor, greater amounts of moisture are driven off keeping the food product relatively cool (i.e., less than browning temperatures) until substantially all moisture is lost. Excessive moisture loss, of course, will render the food product unsuitable for consumption. Moreover, moisture driven from the food product interior by microwave heating tends to soften not crispen the food products exterior surface.
One approach to overcome the problem of the absence of browning and/or crispening during microwave heating has been to develop specialized food packaging for microwave food products. The art directed towards consumer microwave food packaging has experienced tremendous development in the last several years.
While many factors have contributed to the rapid advancement in this art (e.g., increasing preparation of microwave ovens into households, less time available for meal preparation, etc.), perhaps the most significant has been the development of the metallized film microwave susceptor. The metallized susceptor upon exposure to microwave energy can reach temperatures between 400.degree. to 800.degree. F. Such temperatures are high enough to cause browning. When such microwave susceptors are in direct contact with the surface of a food product, upon microwave heating the susceptor quickly heats the food surface. Both the susceptor final temperature and the heating rate are important to obtaining the desired end food product attributes. If either the susceptor temperature or the rate at which the susceptor heats is too low, then moisture from the interior of the food product being heated will continually resupply the moisture being driven off at the food product's surface. As a result, the surface fails to crispen or to reach browning temperatures.
In certain other popular applications such as microwave popcorn products, the microwave susceptor's rapid heating feature is more important than final temperature. The rapid heating allows the fat/popcorn charge to heat more rapidly leading to improved product performance.
The metallized film susceptor in simplest form comprises a plastic film substrate and a thin deposit of a layer of aluminum thereupon. The prior art includes numerous improvements directed towards modifying and improving one or more performance attributes of the metallized films. The prior art includes a wide and rapidly growing variety of packaging structures which employ metallized film microwave susceptors as an integral element of the package.
With the development of this technology, microwave packaging has enjoyed a tremendous increase in popularity and usage in view of the performance, convenience, disposability and low cost of metallized film microwave susceptors. Recently, however, concern has been raised regarding the potential for migration of materials from the metallized film to the foodstuff to be heated. In view of these potential concerns, there is a present and growing need for improved microwave susceptors of enhanced safety but which nonetheless provide the performance, convenience, ease of disposal and cost advantages equivalent to metallized film microwave susceptors.
Another microwave susceptor technology involves chemical compositions which heat upon exposure to microwave energy. Compared to metallized film susceptors, the chemical susceptor art is relatively undeveloped. Most chemical susceptors are fabricated from formulations which are inedible. An early and brief description of an inorganic alumina gel which absorbs microwave energy is given in (Controlled Microwave Heating and Melting of Gels), by Roy et al. (J. Am. Ceram. Soc. 68(7) 392-95, 1985). An improved chemical microwave susceptor composition based upon ceramic oxide gels is described by Seaborne in U.S. Pat. No. 4,806,718, issued Feb. 21, 1989, and is entitled Ceramic Gels with Salt for Microwave Heating Susceptor. The improved chemical microwave susceptor compositions described in the '718 patent include a wider class of ceramic gel-forming materials which themselves are microwave active, and the addition of sodium chloride to improve microwave heating performance. While useful and an advance in the art, these compositions, while not toxic, are not edible per se. Moreover, the compositions upon drying to useful moisture are highly frangible or even particulate in nature. As a result, these microwave active materials must be supported by or formed into useful shapes by an appropriate binder or carrier, e.g., a plastic or a cementitious material.
However, certain chemical microwave susceptors, while not intended to be consumed per se, can be formulated to comprise edible materials. For example, another description of a liquid chemical microwave susceptor composition is given in U.S. Pat. No. 4,283,427 (entitled Microwave Heating Package, Method and Susceptor Composition, issued Aug. 11, 1981 to Winters et al.). The '427 patent teaches that "The chemical susceptor is comprised of a solute, such as inorganic salts of Group IA and IIA, and a polar solvent for the solute such as water." The salts are taught as being useful to reduce the vapor pressure of the polar solvent. The chemical susceptor can also include a "heating profile moderator." A wide variety of materials are taught as being useful to affect one or more heating attributes of the susceptor. The temperature profile materials included are broadly defined and include animal, vegetable and mineral materials. Inasmuch as the chemical microwave susceptor composition is liquid in physical state, the susceptor therein described additionally essentially comprises a holder for the liquid. Moreover, while the chemical susceptor composition is fabricated from materials of low toxicity, the chemical susceptor composition is clearly not intended to be in direct contact with a food item to be heated or to be edible per se.
In a related manner, the prior art further includes a large number of compositions formulated with edible ingredients intended for use as microwave browning compositions. These formulations are chemical susceptors of an edible variety which, however, can obtain temperatures of only about &lt;250.degree. F. In this approach, various sauces and other coatings which absorb or concentrate the microwaves are applied to a food substrate to accomplish heating and/or browning. Exemplary of this is U.S. Pat. No. 5,077,066 (issued Dec. 31, 1991). The general mechanism is to provide browning not by reaching browning temperatures but by including materials that react at lower temperatures to provide the browning effect via the Maillard reaction. Generally, these formulations essentially include the combination of a reducing sugar or reducing agent and a protein or other nitrogen containing material which upon heating react in browning reactions. Alternatively, the compositions comprise colored compositions disguised by microencapsulation. Upon heating, the encapsulant melts to expose the color. These compositions contain edible materials that absorb microwave energy and thus heat. However, such formulations are not intended to generate the high heat or high temperatures (300.degree. F..gtoreq.) of the present invention. Moreover, these coatings have met with only limited commercial success.
The previous efforts to overcome the inability of foods to crispen and brown during microwave cooking or heating are not completely effective in achieving a pleasing crisp texture and a brown color in the microwave oven. In addition, these compositions are not particularly stable for extended periods of time, and this instability typically results in premature browning during storage. There is, therefore, a need for an edible microwave susceptor composition that is shelf-stable and can be activated by microwave energy to produce the distinctive brown color associated with conventional cooking.
Accordingly, it is an object of the present invention to provide an edible microwave susceptor composition.
Another object of the present invention is to provide an edible microwave susceptor that is free of sugar and protein reactants and thus is stable at room temperature for extended periods of time and thus avoids the problems of certain of the prior art compositions.
Another object of the present invention is to provide an edible microwave susceptor that can be applied to the surface of a prepared food substrate that upon microwave heating will heat quickly to over 300.degree. F. thereby rapidly heating the food substrate to crispen and/or brown the surface thereof.
Still another object of the present invention is to provide an edible microwave susceptor composition that achieves heating to temperatures above 300.degree. F.
A further object of the present invention is to provide microwave susceptor compositions that do not require a holding container.
A still further object of the present invention is to provide edible microwave susceptor compositions that can be added to other food compositions for increasing the microwave absorption and heating performance of such fortified food products.
Surprisingly, the above objectives can be realized and superior edible microwave susceptor products provided that quickly heat upon exposure to microwave energy to temperatures exceeding 300.degree. F. The present compositions provide crispening and/or browning benefits to the food products to which they are applied or in which they are incorporated. The compositions are exceptionally stable upon extended storage and require only simple admixture to prepare.